Body color mutants and a transposable element affecting the tyrosinase gene of the medaka fish, Oryzias latipes

Abstract

In the medaka fish Oryzias latipes, many mutants for body colors have been isolated. Among them, albinos (i-locus alleles) are typical col or mutants having amelanotic skin and red -colored eyes with no tyrosinase activity. To cast light on the molecular basis of the albino mecha- nism, we have cloned cDNAs for tyrosinase, a key enzyme in melanin biosynthesis, from the wild- type fish, and performed Southern blot analysis on genomic DNA from an i mutant fish with the cDNA probe. The results demonstrated a 1.9-kb insertion fragment in exon I of the tyrosinase gene. The fragment, designated Tal, (i) carries terminal inverted repeats containing sequences quite similar to those of the TcJ transposable element, (ii) is flanked by duplicated segments of their host chro- mosomes, and (iii) is present in approximately 80 copies in the genome, showing specific features of the TcJ -like transposable element. It is clear that insertion of Tol into the tyrosinase gene causes the albino body color in the i mutant. As a contrast to albino-i alleles, there is a color- less melanophore mutant b, having normally pigmented black eyes but orange-colored skin with amelanotic melanophores, thus showing the presence of a tissue-specific mechanism of mela- nin formation. Using the cDNA probe, Northern blot analysis on poly(AtRNAs from various tissues was carried out. In the wild-type, the tyrosinase mRNA was expressed in eyes and skin but not in liver, corresponding to tissue-specific melanin formation. In the b mutant, contrary to expectation, the mRNA was detected not only in eyes but also in amelanotic skin. Therefore, pigmentation of skin controlled by b is not directly related to expression of the tyrosinase gene.